The present invention is generally related to light-emitting devices and, more particularly, to semiconductor lasers.
The major approach for fabrication of semiconductor lasers operating at a wavelength of 1.3 xcexcm involves the use of strained InAsP multiple quantum wells grown on InP substrates. As for any semiconductor laser, one of the factors that determines the performance of the device is the sensitivity of threshold current to temperature.
The threshold current and emission efficiency of conventional lasers on Inp substrates strongly depend on operating temperature. Specifically, threshold current Ith is defined by the following equation:
Ith=Io exp(T/To)xe2x80x83xe2x80x83(Equation 1)
where Io is a constant, T is the operating temperature, and To is the characteristic temperature. As shown in Equation 1, the threshold current Ith increases exponentially with temperature ratio T/To. Accordingly, if To is a very high value, the threshold current Ith is fairly insensitive to temperature T changes. Therefore, to realize good high temperature performance in lasers, temperature-insensitive characteristics with a high characteristic temperature To are desired.
For the conventional InAsP active region, the temperature performance is limited by the poor electron confinement due to a small conduction band offset between the InAsP of the quantum well (QW) layers and the material of the barrier layers. The performance of these lasers at elevated temperatures is therefore unsatisfactory.
For example, two of the common choices for the barrier material for the InAsP active region are InGaP and AlInGaAs. Although lasers with InAsP QW layers and InGaP barrier layers have low threshold currents, these lasers exhibit poor performance at higher temperatures. Note there is significant amount of disagreement in the scientific community regarding the conduction band discontinuity between InAsP and InGaP.
Lasers with InAsP QW(s) layers and AlInGaAs barrier layers have a higher threshold current than their InAsP/InGaP counterparts but have better characteristics at high temperatures. This may be explained by the existence of defects at the InAsP/AlInGaAs interface. For example, defects which act as non-radiative centers in the QW(s) layers or barrier layers can increase the threshold current and lead to a higher To.
The quality of an epitaxially regrown layer of InAsP also generally improves as the growth temperature is reduced. Therefore, active regions with InAsP QW layers are grown at relatively low temperatures. This low growth temperature, however, does not produce high-quality AlInGaAs barrier layers on the InAsP QW layer material, since the AlInGaAs barrier material requires a higher growth temperature for better crystal quality.
Consequently, the InGaP and AlInGaAs barrier layer materials for InAsP QW layers do not provide desired low threshold currents and high temperature laser performance. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
The present invention provides a laser structure that operates at a wavelength of 1.3 xcexcm at elevated temperatures and a method for fabricating such a laser structure. The laser structure includes a quantum well layer of InAsP. The quantum well layer is sandwiched between a first barrier layer and a second barrier layer. The material of each barrier layer exhibits a higher band gap energy than the material of the quantum well layer. Each barrier layer comprises Gax(AlIn)1xe2x88x92xP in which x 0. This material has a higher bandgap energy than conventional barrier layer materials, such as InGaP. The resulting larger conduction band discontinuity leads to improved high temperature performance without increasing the threshold current of the laser structure.
The present invention also provides a method for fabricating a laser structure that operates at a wavelength of 1.3 xcexcm and at elevated temperatures. The method includes providing a substrate of InP; forming a lower cladding layer over the substrate; forming a first barrier layer of Gax(AlIn)1xe2x88x92xP in which x 0 over the lower cladding layer; forming a quantum well layer of InAsP over the first barrier layer, forming a second barrier layer of Gax(AlIn)1xe2x88x92xP in which x 0 over the quantum well layer; and forming an upper cladding layer over the second barrier layer.